The invention relates to a cylindrical cell which has a cylindrical shape, to a cell pack in which a plurality of the cylindrical cells are connected to each other, and to a cell holder which holds in a bundle a plurality of sealed-type cells.
A nickel-cadmium cell and a nickel-metal hydride cell which are rechargeable are widely employed as a battery power source for a portable apparatus. Such a nickel-cadmium cell or a nickel-metal hydride cell is used mainly in the form of a cylindrical cell which has a cylindrical shape. In a portable apparatus, such a cylindrical cell is used singly or in the form of a cell pack in which a plurality of cells are connected.
As shown in FIG. 10, a cylindrical cell 1 has a structure in which one end of a cylindrical case 12 containing cell materials is closed by a cover 11 via a packing and then caulked. A projection 11a is formed in the cover 11, and a safety valve for releasing gasses is disposed in the projection 11a. In the cylindrical cell 1, generally, the cover 11 having the projection 11a is connected to a positive electrode, and the case 12 which covers the peripheral side face and the bottom face is connected to a negative electrode.
When the cylindrical cell 1 is to be used as a power source for an apparatus, generally, the cell is connected to a power source circuit in the following manner. As shown in FIG. 11, nickel plates 9 of a thickness of about 0.1 to 0.5 mm are spot-welded at one end to the projection 11a and the bottom face of the case 12, respectively. The cell is connected to the power source circuit through the nickel plates 9. When the cylindrical cell 1 is to be used in the form of a cell pack, generally, cells are connected to each other in the following manner. As shown in FIG. 12, two adjacent cells 1 are bridged by a nickel plate 9, and the both ends of the nickel plate are spot-welded to the projection 11a of the cover 11 of one of the cells and the bottom face of the case 12 of the other cell, respectively.
In the case where the cylindrical cell 1 must supply a large current, the resistances of the nickel plates 9 are not negligible, and hence the nickel plates are required to have a large thickness. When the nickel plates 9 have a large thickness, however, it is difficult to spot-weld the plates to the cylindrical cell 1. Therefore, the increase of the thickness of a plate has a limitation.
In the prior art, therefore, the voltage drop and heat generation in the nickel plates 9 are inevitably increased in magnitude when the cylindrical cell 1 supplies a large current, thereby producing a problem in that the cell performance cannot be sufficiently exerted.
A prior art cell holder has a structure such as that shown in Japanese Utility Model Publication (Kokai) No. SHO60-22752. In the cell holder, a plurality of cylindrical cells are arranged in such a manner that their peripheral side faces are adjacent to each other, a frame surrounds the periphery of the arranged cells, and a cover is attached to each of the end faces of the cylindrical cells. The frame consists of a side wall surrounding the periphery of the cells, and a number of partition walls which are inwardly projected from the side wall. The cylindrical cells are supported by inserting them into the spaces between the side wall and the partition walls, respectively. In some cases, a plurality of holes are formed in a block-like frame and cylindrical cells are respectively fitted into the holes, or alternatively a large hole having a shape which is obtained by coupling such holes to each other is formed and a plurality of cylindrical cells are fitted in a bundle into the hole. In the covers, elastic connecting plates are adequately disposed on the faces opposing the both end faces of the cylindrical cells in the frame, so that the cylindrical cells are electrically connected to each other.
In a cell pack using such a cell holder, when required, the peripheral side faces of the cylindrical cells may be covered by a film of a resin, or the like in order to ensure the electrical insulation between the cylindrical cells.
When a cell is discharged, heat is generated as a result of an internal electrochemical reaction and electric resistance. In the case of a heavy current drain, particularly, the amount of generated heat is increased. In a sealed-type secondary cell such as a nickel cadmium cell or a nickel-metal hydride cell, the heat generation occurs also when the cell is charged. When the temperature of the cell is raised as a result of the heat generation, self-discharge may reduce the cell capacity or shorten the service life of the cell. Therefore, it is previously determined that such a cell is used in a temperature range of -10.degree. to 60.degree. C. during a discharging process and 0.degree. to 35.degree. C. during a charging process.
In the prior art cell holder, however, each cylindrical cell is surrounded by the frame and the both ends are covered by the covers so that the interior of the holder is substantially hermetically sealed, and hence heat generated from the cylindrical cells during the discharging or charging process cannot be sufficiently dissipated to the exterior. This produces a problem in that the temperature of the cylindrical cells may be raised to a level higher than the specified range. This problem is also applicable to sealed-type cells of other types such as those of rectangular cells.